Switched capacitor circuits are known and have been used in filter circuits to emulate the behaviour of a resistor by switching charge into and out of a capacitor through switch circuits on either side of the capacitor. They have been used particularly in filter circuits to implement the resistive part of a R-C timing circuit for a filter. That is, the switched capacitor circuit is used to charge up a second capacitor at a particular rate, the rate of charge of the second capacitor depending on the ratio of values between the switched capacitor and the second capacitor. A known switched capacitor circuit comprises a capacitor (the "switched" capacitor) connected between first and second switch circuits. Each switch circuit has two terminals in addition to a terminal connecting it to the switched capacitor. The switch circuit connected to a first terminal of the switched capacitor has a terminal connected to an input voltage for the switched capacitor circuit and a terminal connected to ground. The switch circuit connected to a second terminal of the switched capacitor has a terminal connected to an output of the switched capacitor circuit and a terminal connected to ground. The switch circuits can be controlled by non-overlapping clock signals which cause the switches to toggle between the two terminals. This toggling effect causes charge packets to be transferred into and then out of the capacitor to emulate the behaviour of a resistor.
Subsequent developments have led to the use of switched capacitor circuits in many different structures over and above the emulation of resistor behaviour. These structures behave in a definable manner even though some do not have any continuous time equivalent.
In existing switched capacitor circuits, the required non-overlapping clock signals are supplied to all of the switch circuits in the circuit. This is done by a hard wired bus extending from a clock signal generator to each of the switch circuits. Thus, the switch circuits are committed at the time that the chip is made since at this time the hard wired distribution of the bus is determined.
In some existing circuits the possibility exists to externally program or control the value of components so that the tuning of the circuit can be adjusted. These circuits include programmable switched capacitor filters, tunable transconductor-C circuits and analogue neural networks where synapse weights are adjusted by setting component values. However, all of the known circuits are restricted in being able to implement only a fixed topology and function.
In one aspect, the present invention seeks to provide a switched capacitor circuit which is programmable so that its function can be set by a user.
In another aspect, the present invention seeks to provide a switched capacitor circuit which is field programmable or reprogrammable whereby both the functions of the circuit and the tuning of its response can be set by a user with the circuit in situ.